1. Field of the Invention
The invention relates to a device for displaying color pictures based on an analog video signal of the television type, comprising inter alia a luminance signal and a chrominance subcarrier, said device being provided with a bandstop filter centered on the frequency of the chrominance subcarrier and transmitting the luminance signal, a video signal processing circuit to which the luminance signal from the filter is applied, and a scan velocity modulation arrangement to which the luminance signal from the filter is also applied, said arrangement comprising a circuit for differentiating the luminance signal, followed by an amplifier which feeds a current into a coil, acting on the horizontal deflection of the spot.
Such an arrangement for modulating the scan velocity is intended to give the luminance transitions in a picture a sharper appearance.
With such an arrangement, there may be a risk of applying an improper velocity modulation when parasitic luminance signals, such as noise or a residue of the chrominance, subcarrier are present. Such a residue is always present because it is not possible to sufficiently filter the chrominance subcarrier in the above-mentioned filter because the passband of the luminance signal would then be eroded.
2. Description of the Related Art
An scan velocity modulation arrangement as described in the opening paragraph is known from U.S. Pat. No. 4,309,725. To avoid the above-mentioned drawback, this document describes a circuit in which the amplifier feeding the coil with the luminance signal differentiated by the differentiating circuit is an amplifier having a double threshold (i.e., for positive as well as negative voltage excursions) below which the differentiated signal is not active.
The presence of a threshold is useful, but if this threshold is too high, there will be a risk that useful video signals having a low amplitude are eliminated, whereas the parasitic signals will not be eliminated sufficiently if this threshold is low. A satisfactory compromise is difficult to find.